The invention relates broadly to the field of article grippers and, in particular, the invention relates to tool grippers for tool storage magazines and to article gripper mechanisms such as part loaders which may be used with machine tools.
A variety of article grippers have been used in conjunction with tool handling and part handling machinery, where the tool grippers are often configured as having a pair of independent fingers or jaws which may be opened and closed in a pincer-like movement to grasp the desired article. Some gripper fingers are relatively moved with respect to one another by fluid-powered actuators such as air cylinders. Still other mechanisms make use of a spring device to bias the jaws toward one another to thereby grip the desired article.
Certain prior art devices have been constructed from injection molded plastic, where the plastic has an inherent spring rate and may be deflected through a range of movement to permit part entry and exit. One such prior art device is described in U.S. Pat. No. 4,858,980, of Dreisig et al, assigned to Cincinnati Milacron Inc., the assignee of the present invention. In this reference, a movable jaw is supported on flexible reed springs which are integral with a gripper base which supports a fixed jaw.
Still another prior art device, German Reference DE3511470A1, provides for a one-piece plastic gripper, having jaw portions which may be spread from their normal position to permit entry and exit of a workpiece.
One difference between the '980 patent and the German reference is the '980 patent provides for different loads to be seen by the workpiece during entry and exit, whereas in the German reference the spring force is substantially at 90.degree. to the entry and exit path of the workpiece and, thus, the load to deflect the two beam-like jaw portions is the same whether a workpiece is entering or exiting the gripper. Both references have fixed spring loads.
The inventors of the present invention have studied the inherent problems and advantages of the prior art devices and have determined that modern requirements for machine tools, in particular. dictate that low cost be an objective in the overall design of the device.
The inventors have determined that a force-supplying means such as a spring and bolt combination acting at 90.degree. to the part entry and exit path will yield an easily-adjustable means for varying the clamp force of the gripper, and thus need not rely on the inherent springyness of the material from which the jaws are made. This freedom permits greater latitude in jaw design. It will be noted that, for parts made from engineering grade plastics, initial preloads and clamping loads held for long periods of time may tend to dissipate as the material relaxes and the phenomenon of creep takes over.
Accordingly, it is preferred that an article gripper include a stable spring device which is adjustable and which will hold its set load for substantial periods of time; and, for a plastic gripper, without the plastic constraint exhibiting creep.